<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>

    <style>
        body{
            margin: 0;
            padding: 0;
            text-decoration: none;
        }

        #app{
            width: 300px;
            border: 1px solid red;
            margin: 0 auto;  /*外边框的妙用：居中  第一个0代表上下，第二个auto代表左右*/
        }

        h2{
            text-align: center;
            background: brown;
            margin-top: 0;
            margin-bottom: 0;
        }

        form{
            background: aqua;
        }

        input{
            border: 1px solid black;
        }

    </style>

</head>
<body>

<div id="app">
    <h2>会员登录</h2>
    <form action="">
        <div>
            <span>用户名：</span>
            <input type="text">
        </div>
        <div>
            <span>密码：</span>
            <input type="password">
        </div>
        <div>
            <span>邮箱：</span>
            <input type="text">
        </div>
    </form>
</div>

</body>
</html>